1. Field of the Invention
The invention relates to a control unit for an electromagnetically driven valve. More specifically, the invention relates to a control unit which controls an electromagnetically driven valve of an internal combustion engine such that a drive valve is moved to a desired position.
2. Description of the Related Art
A conventional type of electromagnetically driven valve is disclosed in, for example, U.S. Pat. No. 6,467,441. U.S. Pat. No. 6,467,441 discloses a rotary driven type electromagnetic actuator in which a valve of an internal combustion engine operates by using both electromagnetic force and elastic force of a spring. The electromagnetic actuator includes a valve having a stem, and an oscillating arm having a first end portion that is movably supported by a support frame and a second end portion that contacts an end of the stem.
An electromagnet formed of an iron core and a coil wound around the iron core is provided on each of both sides of the oscillating arm. The electromagnetic actuator further includes a torsion bar that is provided at the first end portion of the oscillating arm and that applies force for moving the valve to the valve opening position, and a spiral spring that is provided at the outer periphery of the stem and that applies force for moving the valve to the valve closing position. The oscillating arm oscillates using the first end portion as a supporting point due to the elastic force of the torsion bar and the elastic force of the spiral spring. The movement of the oscillating arm is transferred to the stem via the second end portion, causing the valve to reciprocate between the valve opening position and the valve closing position.
An operation method of an electromagnetically driven valve is disclosed in, for example, Japanese Patent Application Publication No. 09-195736 A. Japanese Patent Application Publication No. 09-195736 A discloses the operation method of an electromagnetically driven valve, for improving a response to the operation, reducing electric power consumption, and realizing an operation suitable for a supply voltage. According to the operation method of an electromagnetically driven valve, a plunger is moved, due to electromagnetic force, from the neutral position, at which a balance is kept between the forces of springs provided on both sides of the plunger connected to a valve, in a first direction or a second direction that is opposite to the first direction. The operation method includes a starting step, a holding step, and an operating step. In the starting step, a self-excited oscillation of the plunger is caused due to electromagnetic force formed based on a first current value. In the holding step, the plunger whose amplitude has been increased due to the self-excited oscillation is attracted in the first direction and then kept attracted in the first direction due to electromagnetic force formed based on a second current value that is smaller than the first current value. The operating step includes a process in which the plunger that has been attracted in the first direction is moved in the second direction, or the plunger that has been attracted in the second direction is moved in the first direction due to electromagnetic force formed based on a third current value; a process in which the plunger is attracted in the second direction or in the first direction while the current value decreases from the third current value to a fourth current value that is smaller than the third current value; and a process in which the plunger is kept attracted in the second direction, or the plunger is kept attracted in the first direction due to electromagnetic force formed based on the fourth current value.
According to an operation method disclosed in U.S. Pat. No. 6,467,441, when the step is shifted from the starting step to the operating step, the plunger is moved with high response. In the operating step, the plunger is gently attracted in the first or second direction, and, therefore, electric power consumption can be suppressed.
Japanese Patent Application Publication No. 09-195736 A discloses a parallel driven type electromagnetically driven valve in which two electromagnets are arranged in the axial direction of a valve stem. Accordingly, the height of the electromagnetically driven valve becomes high, making it difficult to satisfy a requirement concerning a height of the electromagnetically driven valve to be mounted in an engine when the electromagnetically driven valve is used as an intake/exhaust valve of the engine for a vehicle or the like. Concerning the rotary driven type electromagnetic actuator disclosed in U.S. Pat. No. 6,467,441, the height of the actuator is reduced to some extent by providing the torsion bar at the second end portion of the oscillating arm. However, as in the case of the electromagnetically driven valve disclosed in Japanese Patent Application Publication No. 09-195736 A, since the spiral spring is provided at the outer periphery of the stem, the height of the actuator cannot be made sufficiently low.
In the electromagnetic actuator disclosed in U.S. Pat. No. 6,467,441, two electromagnets are arranged in the axial direction of the stem. Similarly, in the electromagnetically driven valve disclosed in Japanese Patent Application Publication No. 09-195736 A, two electromagnets are arranged in the axial direction of the valve stem. The electromagnets one of which is provided for opening the valve and the other of which is provided for closing the valve contribute an increase in the height of the electromagnetic actuator or the electromagnetically driven valve.
Accordingly, it is considered that, for example, one of the two electromagnets is omitted in order to reduce the height of the electromagnetically driven valve. In this case, however, even if the operation method of the electromagnetically driven valve disclosed in Japanese Patent Application Publication No. 09-195736 A is applied to the case where only one electromagnet is provided, the drive valve cannot be controlled to a desired position.
The European patent application EP 1 209 328 A2 further discloses a control method for an electromagnetic actuator for the control of an engine valve in which at least one electromagnet displaces an actuator body under the action of the force of magnetic attraction generated by the electromagnet, the electrical supply of the electromagnet being controlled as a function of an objective value of the magnetic flux circulating in the magnetic circuit formed by the electromagnet and the actuator body.
Further, the European patent application EP 1 098 072 A1 discloses a method for the control of electromagnetic actuators for the actuation of intake and exhaust valves in internal combustion engines, wherein an actual position and an actual velocity of the valve as well as a reference position and a reference velocity are determined and the differences between the reference position and the actual position as well as between the reference velocity and the actual velocity of the valve are minimized by means of a feedback control action.